Gangarajula Sudhakar

Total Synthesis and Stereochemical Revision of Acortatarins A and B

A first total synthesis of acortatarins A, B, and an enantiomer of the proposed structure of acortatarin B is described by using readily available d-sugars. This convergent total synthesis revealed the revision of the absolute configuration of acortatarin A and structural revision of acortatarin B. The key steps involved are regioselective epoxide opening with deprotonated 2,5-disubstituted pyrrole and spiroketalization.

Nazarov Cyclization of Divinyl and Arylvinyl Epoxides: Application in the Synthesis of Resveratrol-Based Natural Products

New variation in the Nazarov cyclization has been developed by preparing divinyl and arylvinyl epoxides as pentadienyl cation precursors for the first time. Highly substituted cyclopentadienes, hydrindienes, and indenes were synthesized to demonstrate the compatibility of this reaction with substrates bearing a variety of substitutions and having different types of epoxides. Application of this method in the synthesis of resveratrol-based natural products was also demonstrated.

Total Synthesis and Absolute Configuration of Curvularides A-E

The first total synthesis of curvularides A-E, isolated from a culture broth of the endophytic fungus Curvularia geniculata, is described. The divergent total synthesis reported herein confirmed the absolute configurations of curvularides A-E and supported that these natural products might be obtained from a common biosynthetic pathway. The key steps involved in the synthesis were the diastereoselective hydrogenation of exo-methylene-γ-butyrolactone to α-methyl-γ-butyrolactone, Sharpless kinetic resolution, Sharpless asymmetric epoxidations, and intramolecular and intermolecular epoxide openings.

Total synthesis of varitriol, varioxirane, and enantiomer of the proposed biosynthetic precursor.

The first stereoselective total synthesis of varioxirane was accomplished, and the proposed biosynthetic pathway was supported by converting varioxirane to (+)-varitriol. The first total synthesis of enantiomer of the proposed biosynthetic precursor, (1E,3S,4R,5E)-1-(2-(hydroxymethyl)-3-methoxyphenyl)hepta-1,5-diene-3,4-diol, was also achieved by utilizing the unreacted allylic alcohol obtained during the Sharpless kinetic resolution step. Other key steps include the Horner-Wadsworth-Emmons reaction and the diastereoselective reduction of α,β-unsaturated ketone to its corresponding alcohol.

Nazarov Cyclization and Tandem [4 + 2]-Cycloaddition Reactions of Donor–Acceptor Cyclopropanes

The development of aryl vinyl/divinyl donor-acceptor cyclopropanes (DACs) as novel Nazarov cyclization (NC) precursors is described. The 1,3-zwitterion, generated from DACs embedded in the divinyl framework, acts as a pentadienyl cation, a requisite for Nazarov cyclization. A cyclic allyl cation in the course of NC was trapped with external nucleophiles to provide an interrupted NC product. Indeed, an allyl cation in this reaction is analogous to a 1,4-zwitterion that on reaction with dipolarophiles provided an easy access to substituted pyrans with excellent yield and diastereoselectivity via NC followed by a formal [4 + 2] cycloaddition.